The present invention generally relates to data communications, and more particularly to systems and methods for simultaneously measuring the phase and amplitude jitter impairments and the signal to noise ratio of a quadrature amplitude modulated data communication channel.
It has long been desired to measure the line impairments in a data communication system employing quadrature amplitude modulation (QAM). Indeed, many inventions have been directed toward the same, including, e.g., U.S. Pat. No. 4,381,546 to Armstrong; U.S. Pat. No. 3,924,188 to Hofbauer; and U.S. Pat. No. 4,555,790 to Betts et al., which among them portend to solve for phase jitter, amplitude jitter, noise, harmonic distortion, and frequency offset in different manners.
Among the various line impairments, it would be especially advantageous to solve simultaneously for phase jitter, amplitude jitter, and noise. However, as is readily appreciated, in QAM systems only two pieces of data (the x and y coordinates of a received point) are measurable from which the line impairment information may be gleaned. Thus, only two known variables are available for solving three simultaneous equations. The prior art, including the above-cited patents, have responded to this problem by either ignoring all other impairments in solving for two impairments simultaneously, or, as in the Armstrong U.S. Pat. No. 4,381,546, opted to solve for more than two impairments in a separate manner. When solving separately, all the error is assigned to a particular impairment even the error might be the result of another impairment. Neither approach is optimal, and the fact that this problem is particularly troublesome to the art is evidenced by U.S. Pat. No. 4,555,790 to Betts et al. which states that "[since] for signals located relatively far from the origin the (error) is due to noise, . . . (and) phase and amplitude errors, the (Betts, et al. invention) is adapted to respond only to . . . signals . . . where the effects of . . . phase and amplitude errors and negligible". Indeed, this is good evidence that none of the known art can solve simultaneously in an accurate manner for three impairments when all three are simultaneously present in the QAM system.